I have two hybrids: one low-end for winter and rain, and another nicer one - based on a fast road bike - for nice weather. I ride one of them between 16 and 20 miles almost every weekday, plus some weekends. Part road, part multi-use trail, part (gasp!) sidewalk designated as bike trail.
I find hybrids perfect for my kind of riding, and far better than the heavier and out-of-place moutain bikes that I pass as they labor along. I often wonder if they realized that they bought the wrong kind of bike for the riding they do.
However, I only occasionally go fully offroad -- the longest has been a three hour ride in the hills. If I were gonna do that often or do longer rides, I would definitely want a mountain bike but never would use the heavier mountain bike for around-the-town crusing. Waste of effort lugging the useless gear along, and less fun because they are pokier.